The present invention relates generally to the art of hydraulics and more specifically relates to a system for varying the displacement of reciprocating piston type pumps.
In the past there have been many proposals for varying the displacement of reciprocating piston type pumps. These include both manual systems for varying the displacement at the will of an operator and automatic systems for varying the displacement to obtain substantially constant pressure. The proposals have included varying the eccentricity of a cam or other piston reaction member, varying the angular relationship of a swash plate or equivalent piston reaction member with respect to the pistons, controlling the quantity of fluid flowing to the pistons on their intake stroke, and controlling intake stroke length by maintaining either a constant inlet pressure or a constant drive chamber pressure and varying the drive chamber pressure or the inlet pressure respectively.
Each of the previous proposals has its advantages and disadvantages and in many cases the proposal used depended not only on the particular application but also on personal preference. Proposals for varying the eccentricity of a cam or the angularity of a swash plate have the disadvantage of being somewhat complicated, but have the advantage of providing accurate displacement control. These last-mentioned proposals were also speed sensitive. That is, the pump flow or output would vary with the speed at which the pump was driven. Depending on the application, this was either an advantage or disadvantage.
The previous proposals for varying displacement by controlling the quantity of fluid flowing to the pistons on their intake stroke have the advantage of being relatively simple, but for some applications have the disadvantage of not being speed sensitive.
Previous proposals for varying displacement by maintaining either a constant inlet pressure or a constant drive chamber pressure and varying the drive chamber pressure or inlet pressure, respectively, have the advantage of relative simplicity, but, in those applications requiring speed sensitivity, it was extremely difficult to obtain accurate control of displacement over the entire range of displacement.